1. Field of the Invention
The present invention relates to semiconductor devices, in particular, having structures in which substrates composed of semiconductor materials different from each other are bonded together, and manufacturing methods thereof.
2. Description of the Related Art
Recently, in order to realize integration of semiconductor materials of different kinds such as gallium arsenide (GaAs) and indium phosphide (InP) in a semiconductor device which uses, for example, compound semiconductor materials, technology of directly bonding together semiconductor substrates which use these materials has been utilized.
The technology of bonding substrates as described above is disclosed, for example, in Japanese Patent Application Laid-open No. 61-183915 and a paper, R. J. Ram et al., J. Appl. Phys. Vol. 78, 4227 (1995). In this technology, substrates are bonded in a manner in which surfaces of the substrates are compressively bonded with each other after being subjected to cleaning treatment and in this state, are subjected to thermal treatment in a reducing atmosphere (for example, hydrogen).
FIG. 6A is a diagrammatic sectional view showing a state in which a semiconductor substrate 11 and a semiconductor substrate 12 are bonded with each other according to this method. In the conventional bonding method, annealing time at bonding temperature is set very short (for example, approximately 30 minutes at a temperature of approximately 450° C.) due to a fear of deterioration in crystallinity. A region including an interface of the semiconductor substrate 11 and the semiconductor substrate 12 is structured to include structural contact between substrates to form atomic level bonding, as shown in FIG. 6A.
In this way, when the substrates are brought into mutual contact and bonded together, there is no need to consider lattice matching of materials of the two substrates. This makes it possible to perform the bonding operation without being restricted by kinds of semiconductors. Furthermore, defects in crystallinity are limited only in the vicinity of the interface so that crystallinity with a small amount of defects is obtainable in other regions.
However, when the substrates are brought into mutual contact and bonded together, substrates with different lattice constants are forced to be bonded. In this case, since thermal expansion coefficients of semiconductors of different kinds are different from each other, a problem occurs that stress is generated in the bonding interface when the substrates are thermally treated for bonding. This is because the thermal treatment in the bonding process causes constituent atoms of the substrates to move and be combined with each other on the interface on an atomic level, and thereafter, the stress due to the combination including different lattice constants is preserved.
An atom rearranging layer including a stress is used for the bonding, for example, in Japanese Patent Application Laid-open No. 5-267790. With this stress existing on the interface, when a light-emitting layer such as a quantum well is arranged in the vicinity of the interface, the light-emitting layer is influenced by the stress to deteriorate the crystallinity. Consequently, the deterioration of the crystallinity lowers light-emitting efficiency. Particularly, when the bonding interface is applied to a surface emitting type laser, the light-emitting efficiency lowers greatly since the surface emitting type laser has a structure in which the light-emitting layer is very close to the interface.
As means for reducing the stress, Japanese Patent Application Laid-open No. 6-349692 discloses that an intermediate bonding layer is formed on the bonding interface of semiconductor substrates of different kinds. However, when this technology is to be applied to, for example, the surface emitting type laser, forming the intermediate bonding layer affects the film thickness of an epitaxial layer in a vertical direction to the substrates to give influence to a device characteristic, which results in restriction of device design. Moreover, since the formation of the intermediate bonding layer necessitates a new process, a problem that a whole process is complicated arises.
Furthermore, a problem occurs in an electrical characteristic of the bonding interface. This is a problem that a current-voltage characteristic becomes nonlinear since an energy band changes discontinuously due to a steep characteristic of the bonding interface.
FIG. 6B is a graph showing a discontinuous state of the energy band due to the steep characteristic of the interface. In the bond shown in FIG. 6A, an energy barrier is formed and an energy change on the interface is in a discontinuous state so that the energy band changes discontinuously between the semiconductor substrate 11 and the semiconductor substrate 12. As the bonding on the atomic level is performed more appropriately, an electrical barrier for an electron becomes bigger. When this technology of substrate bonding is applied to a device such as the surface emitting type laser, a current crosses the bonding interface to cause an increase in driving voltage.
Moreover, the steep change of the energy band causes the current-voltage characteristic to be nonlinear as shown in FIG. 6C. The nonlinear current-voltage characteristic on the interface causes a problem of great degeneration in controllability of an element.
For example, when the bonding interface having this nonlinear current-voltage characteristic is applied to an optical device such as a light-emitting element, it is very difficult to control light-emitting quantity thereof and weak light emitting cannot be performed. In addition, when it is applied to a photodetector, a problem of degeneration in detection accuracy is caused.
As for the nonlinearity of an electrical characteristic, Japanese Patent Application Laid-open No. 6-90061 discloses that a linear current-voltage characteristic is obtainable. However, it is impossible to avoid the problem as described above that an optical characteristic is ruined when the light-emitting layer is near the interface.
As described above, the conventional technology of substrate bonding is not capable of solving both of the problems of the deterioration in the optical characteristic in the vicinity of the interface and of the nonlinearity of the current-voltage characteristic on the interface at the same time.